<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #result {
            width: 300px;
            height: 100px;
            border: solid 1px #903;
          }
    </style>
</head>
<body>
    <div id="result"></div>
    <button>点击发送请求</button>
    <script>
        const btn = document.getElementsByTagName('button')[0];
        const result = document.querySelector('#result');

        //绑定事件
        btn.addEventListener('click',function(){
            //创建
            const xhr = new XMLHttpRequest();
            //超时设置
            xhr.timeout = 2000;
            //超时的回调
            xhr.ontimeout = function(){
                alert("网络异常，请稍后重试！！！");
            }
            //网络异常回调
            xhr.onerror = function(){
                alert("你的网络似乎出现了一些问题！");
            }
            xhr.open('GET','http://127.0.0.1:8000/delay');
            xhr.send();
            xhr.onreadystatechange = function (){
                if(xhr.readyState === 4){
                    if(xhr.status >= 200 && xhr.status < 300){
                        //返回服务器返回的结果
                        result.innerHTML = xhr.response;
                    }
                }
            }
        })
    </script>
</body>
</html>